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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 27, issue 1
Ann. Geophys., 27, 185–197, 2009
https://doi.org/10.5194/angeo-27-185-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 27, 185–197, 2009
https://doi.org/10.5194/angeo-27-185-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  13 Jan 2009

13 Jan 2009

The presence of large sunspots near the central solar meridian at the times of major geomagnetic storms

D. M. Willis1,2, R. Henwood3, and F. R. Stephenson4 D. M. Willis et al.
  • 1Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
  • 2Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, Coventry CV4 7AL, UK
  • 3UK Solar System Data Centre, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
  • 4Department of Physics, University of Durham, Durham DH1 3LE, UK

Abstract. A further study is made of the validity of a technique developed by the authors to identify historical occurrences of intense geomagnetic storms, which is based on finding approximately coincident observations of sunspots and aurorae recorded in East Asian histories. Previously, the validity of this technique was corroborated using scientific observations of aurorae in Japan during the interval 1957–2004 and contemporaneous white-light images of the Sun obtained by the Royal Greenwich Observatory, the Big Bear Solar Observatory, the Debrecen Heliophysical Observatory, and the Solar and Heliospheric Observatory spacecraft. The present investigation utilises a list of major geomagnetic storms in the interval 1868–2008, which is based on the magnitude of the AA* magnetic index, and reconstructed solar images based on the sunspot observations acquired by the Royal Greenwich Observatory during the shorter interval 1874–1976. It is found that a sunspot large enough to be seen with the unaided eye by an "experienced" observer was located reasonably close to the central solar meridian for almost 90% of these major geomagnetic storms. Even an "average" observer would easily achieve a corresponding success rate of 70% and this success rate increases to about 80% if a minority of ambiguous situations are interpreted favourably. The use of information on major geomagnetic storms, rather than modern auroral observations from Japan, provides a less direct corroboration of the technique for identifying historical occurrences of intense geomagnetic storms, if only because major geomagnetic storms do not necessarily produce auroral displays over East Asia. Nevertheless, the present study provides further corroboration of the validity of the original technique for identifying intense geomagnetic storms. This additional corroboration of the original technique is important because early unaided-eye observations of sunspots and aurorae provide the only possible means of identifying individual geomagnetic storms during the greater part of the past two millennia.

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